1. Field of the Invention
The present invention relates to a dielectric filter which is preferably used in antenna duplexers for mobile telecommunications.
2. Description of the Prior Art
As conventional technique in this field, the following examples are known: Japanese patent laid-open publication No. 108302/1990 applied for in the name of the same applicant as that of the present application: and U.S. Pat. No. Re. 32,768 assigned to Motorola Inc.
FIG. 1 is a perspective view showing a first example of the conventional dielectric filter that adopts the technique of U.S. Pat. No. Re. 32,768. The filter is arranged in such a manner that one stage of a notch filter is inserted which has an attenuation pole at the lower or upper frequency band of the passband. As shown in FIG. 1, the dielectric filter has five independent blocks of dielectric material 51-1-51-5, the height of which is represented by the letter H. The dielectric blocks 51-1-51-5 have dielectric resonators 52-1-52-5 vertically embedded thereinto in a parallel fashion, each of which dielectric resonator is composed of a cylindrical central conductor. The bottom ends of the central conductors are connected to electrically conductive, metallized patterns formed on the bottom surfaces 53-1-53-5 of the dielectric blocks 51-1-51-5, respectively. The metallized patterns on the bottom surfaces are further connected to metallized patterns on the sides (hatched portions in FIG. 1) of the dielectric blocks 51-1- 51-5. In addition, at the top surfaces 54-1-54-5, the central conductors of the dielectric resonators 52-1-52-5 are soldered to conductive wires 55-1-55-5 extending to a board 56, respectively. The sides of dielectric blocks 51-1-51-5 are joined together by soldering so that they are integrally arranged as shown in FIG. 1.
The board 56 is provided so that it faces the top surfaces (open surfaces) 54-1-54-5 of the dielectric blocks 51-1-51-5. Although the bottom surface and sides of the board 56 are not metallized, the top surface (open surface) 57 is provided with conductive patterns 58-1-58-5 for adjusting coupling amounts. The coupling amount adjusting patterns 58-1-58-5 are soldered to the conductive wires 55-1-55-5, respectively. Furthermore, the top surface (open surface) 57 of the board 56 is provided with an input pattern 59-1 for receiving an input signal, and an output pattern 59-2 which is located between the coupling amount adjusting patterns 58-4 and 58-5. An input pin 60-1 and an output pin 60-2, which are made of electrically conductive wires, are soldered to the input pattern 59-1 and output pattern 59-2, respectively.
FIG. 2 shows an equivalent circuit of the first example of the conventional dielectric filter shown in FIG. 1. In FIG. 2, parallel resonance circuits (l.sub.52-1, C.sub.52-1), (l.sub.52-2, C.sub.52-2), (l.sub.52-3, C.sub.52-3), (l.sub.52-4, C.sub.52-4), (l.sub.52-5, C.sub.52-5) correspond to inductances and capacitances of dielectric resonators 52-1-52-5, respectively. Reference indication C.sub.59-1 denotes a capacitance between the input pattern 59-1 and the coupling amount adjusting pattern 58-1; representation C.sub.59-2 denotes a capacitance between the output pattern 59-2 and the coupling amount adjusting pattern 58-5; indication C.sub.58-1 similarly denotes a capacitance between the coupling amount adjusting pattern 58-1 and the coupling amount adjusting pattern 58-2; reference indication C.sub.58-2 denotes a capacitance between the coupling amount adjusting pattern 58-2 and the coupling amount adjusting pattern 58-3; indication C.sub.58-3 denotes a capacitance between the coupling amount adjusting pattern 58-3 and the coupling amount adjusting pattern 58-4; and indication C.sub.58-4 denotes a capacitance between the coupling amount adjusting pattern 58-4 and the output pattern 59-2. It is seen from FIG. 2 that the attenuation amount of the filter in the passband tends to decline as the frequency increases because the dielectric resonators are capacitively coupled, and that the notch filter circuit is composed of a series resonance circuit of C.sub.59-2 and L.sub.52-5. Thus the notch frequency of the notch filter circuit is the series resonant frequency of C.sub.59-2 and L.sub.52-5.
FIG. 3 illustrates the attenuation characteristic of the dielectric filter of the first conventional example with its notch frequency at the lower frequency band of the passband. As seen from this figure, the notch frequency at the lower frequency band of the passband enables the attenuation amount to take a large value, which in turn can reduce the number of stages of the notch filter circuit, resulting in a reduction in the size of the filter. In contrast, when the notch frequency is located at the upper frequency band of the passband, one stage of a notch filter circuit is not sufficient to achieve a desired attenuation amount because of the capacitive coupling. Therefore, the number of stages of the notch filter circuit must be increased, thereby increasing the size of the filter.
FIG. 4 is a perspective view showing a second example of a conventional dielectric filter. In FIG. 4. reference numeral 71 designates a unitarily constructed dielectric block whose width is represented by W, length is L, and height is H. On the front surface (front side) 80, back surface (back side) 81, left side 82, right side 83 and bottom surface 84 of the dielectric block 71 are formed conductive, metallized surfaces by plating, for example. On the top surface (open surface) 85 of the dielectric block 71, conductive frequency adjusting patterns 72-1-72-4 are formed between which holes 73-1-73-3 are provided. In the dielectric block 71, central conductors 74-1-74-4 which function as dielectric resonators (and hence are called dielectric resonators hereinafter) are embedded through the frequency adjusting patterns 72-1-72-4, respectively. Moreover, in the dielectric resonators 74-1-74-4, external circuits 75-1-75-4 are embedded (which are separately depicted above the dielectric block 71 in FIG. 4 for convenience of explanation). The external circuits 75-1 and 75-2 connect the dielectric resonators 74-1 and 74-2, and the external circuits 75-3 and 75-4 connect the dielectric resonators 75-3 and 75-4 so that those external circuits form portions of the attenuation poles. The external circuits 75-1-75-4 are composed of the following elements: cylindrical portions 78-1-78-4 which are made of a dielectric material such as a glass epoxy resin with a diameter of D, and are inserted into the dielectric resonators 74-1-74-4, respectively: an input pin 76-1 formed on the top surface of the cylindrical portion 78-1; coupling pins 77-1 and 77-2 formed on the top surfaces of the cylindrical portions 78-2 and 78-3, respectively: an output pin 76-2 formed on the top surface of the cylindrical portion 78-4; a conductive wire 79-1 connecting between the input pin 76-1 and the coupling pin 77-1; and an electrically conductive wire 79-2 connecting between the coupling pin 77-2 and the output pin 76-2. The diameter of the pins is represented by d (&lt;D). The insides of the holes 73-1-73-3 are not plated by any conductive, metallized layer, 14 being only simple holes with certain diameters and depths. Those diameters and depths are varied to adjust the coupling amounts between the dielectric resonators. The holes for the dielectric resonators and the holes for adjusting the coupling amounts are formed nearly parallel.
FIG. 5 is an equivalent circuit of the dielectric filter of FIG. 4. In that equivalent circuit, the inductance of the conductive wire 79-1 connecting the input pin 76-1 and the coupling pin 77-1, and that of the conductive wire 79-2 connecting between the output pin 76-2 and the coupling pin 77-2 are neglected because they are small.
In FIG. 5, notations (l.sub.74-1, C.sub.74-1), (l.sub.74-2, C.sub.74-2), (l.sub.74-3, C.sub.74-3), and (l.sub.74-4, C.sub.74-4) designate inductances and capacitances of the dielectric resonators 74-1-74-4, respectively. Indication C71 represents the capacitance between the input pin 76-1 and the dielectric resonator 74-1, and C75 represents the capacitance between the output pin 76-2 and the dielectric resonator 74-4. Represetation l.sub.72 denotes the inductance between the dielectric resonators 74-1 and 74-2, which is controlled by adjusting the hole 73-1, l.sub.73 denotes the inductance between the dielectric resonators 74-2 and 74-3, which is controlled by adjusting the hole 73-2, and l74 denotes the inductance between the dielectric resonators 74-3 and 74-4, which is controlled by adjusting the hole 73-3. Reference indication C.sub.77 represents the capacitance between the coupling pin 77-1 and the dielectric resonator 74-2, and C.sub.78 represents the capacitance between the coupling pin 77-2 and the dielectric resonator 74-3. The equivalent circuit shows that the attenuation poles ft.infin. exist in the upper frequency band of the passband. The attenuation poles ft.infin. can be expressed as ##EQU1##
FIG. 6 illustrates the attenuation characteristic of the second example of a conventional dielectric filter. This figure shows that the attenuation pole frequency ft.infin. exists at the upper frequency band of the passband. This type of dielectric filter has the following two problems arising from using the holes for adjusting the coupling between the dielectric resonators: one is that its size cannot be made small; and the other is that the coupling adjustment is difficult.
In short, the conventional dielectric filters present the following problems: The first type of dielectric filter which uses the notch filter to form the attenuation pole frequency at the lower frequency band of the passband has the problem that its size cannot be made small because one stage of a notch filter provides only insufficient attenuation, and hence an increasing number of stages of the notch filter must be provided.
The second type of dielectric filter which uses the holes to form the attenuation pole frequency at the upper frequency band of the passband has the problem that the adjustment of the coupling amounts is difficult and takes a time, in addition to the fact that its size cannot be made small. This is because the holes of more than a certain limited diameter are required, and the coupling amounts must be adjusted by changing positions, diameters, and depths of the holes.